Field of Invention
This invention relates to controlled ignition of combustible fluid materials or fluid mixtures in a closed environment, and in particular to a laser ignition device for an internal combustion engine constructed from high power high brightness semiconductor lasers including VCSEL devices, and is suited to be operable under high mechanical, vibrational and thermal stress.
Related Background Art
Ignition of a combustible fluid or a combustible fluid mixture such as, gasoline and air, etc. (combustion fuel hereinafter) in an internal combustion engine using conventional electrical spark plugs, is inefficient and therefore uses more fuel and generates more emission gases that cause pollution. A major advantage of improving combustion efficiency would result in using leaner combustion mixture (having a low fuel/air ratio) to support nearly complete combustion which would thereby facilitate fuel efficiency and lower environmental cost. While a lot of research and resource is directed towards improving combustion efficiency, none has proven better than using an ignition device and a process that will allow use of a leaner combustion mixture and ignition at higher pressure.
The concept of using a high intensity light source for reliable and efficient combustion as compared to using a conventional electrical spark plug has been described in the U.S. Pat. No. 3,861,371 issued on Jan. 21, 1975 to Gamell. The light source in this approach is a high intensity broad band light source, such as a high intensity flash lamp or a strobe light. As the technology progressed, the high intensity light source was replaced by a laser source, typically a solid state laser, for example a ruby or a Nd:YAG laser. However, to achieve high enough output power using traditional optical pumping schemes require a very large pumping source and cooling device. In recent years, different innovative concepts have been applied to facilitate a better ignition device and a better combustion process to facilitate a near complete combustion of the fuel mixture.
These innovative concepts include using optical fiber to deliver a continuous wave (CW) or pulsed laser radiation to the combustion chamber, high energy Q-switched pulsed laser operation, ricocheting the laser beam in the combustion chamber by positioning reflecting surface within the ignition chamber, using a plurality of CW or pulsed laser beam to focus at different regions of the combustion chamber to ignite the combustion fuel in different regions. as has been described in many patent and non-patent literature publications (U.S. Pat. No. 4,314,530 issued on Feb. 9, 1982, to Giacchetti, U.S. Pat. No. 4,416,226 issued on Nov. 22, 1983 to Nishida et al., United States Patent Application Publication No. 20140238038 published on Aug. 28, 2014, by Macchia et al.).
A wide range of laser devices including solid state lasers, edge and surface emitting semiconductor lasers are used to construct ignition devices. The laser light is directly launched in the combustion chamber through a suitable transparent window that seals the laser apparatus from the combustion chamber and thereby impervious to the combustion process. In addition, one or more optical elements are used to achieve a desired beam profile and quality so as to deliver high energy laser pulses directly in to the combustion chamber through the window. Therefore, it is extremely important to incorporate a device for example a pre-chamber for generating a high temperature plasma or/and a method, to keep the window clean of combustion fluid mixture or residue from the combustion process to minimize attenuation of the laser light during operation of the engine as has been described in the U.S. Pat. No. 7,806,094 issued on Oct. 5, 2010 to Gruber.
One important requirement for reliable operation of an optical ignition device is optical beam quality and optical power to efficiently burn a combustion mixture, and mechanical and thermal robustness to survive mechanical, vibrational and thermal stress of an internal combustion engine environment. In addition, it is desirable to have a form factor that closely matches the form factor of a conventionally used electrical spark plug. An ignition device using VCSELs as ignition source is described in the United States Patent Application Publication No. 20130291818 published on Nov. 7, 2013, by Hartke et al. There it describes a laser spark plug which uses a plurality of VCSELs to generate multiple beams that are focused using optical components to ignite combustion fluid mixture (vapor) at plurality of regions in an internal combustion engine.
A more widely used configuration of laser ignition device includes an optically pumped solid state laser medium. Optical pumps include semiconductor lasers for example, edge emitting or surface emitting lasers including VCSEL or arrays of semiconductor lasers. Due to the small size as well as superior emission properties of VCSELs or VCSEL arrays, laser ignition devices in a small form factor external housing may be installed using a threaded section to an external wall of a combustion chamber similar to a conventional electric spark plug (United States Patent Application Publication No. 20140109855 published on Apr. 24, 2014, by Gruber, United States Patent Application Publication No. 20140216383 published on Aug. 7, 2014, by Nuebel et al., United States Patent Application Publication No. 20130276738 published on Oct. 24, 2913 by Hartke).
Small form factor optical pumps using semiconductor edge emitting lasers or VCSELs are also suitable for pumping solid-state laser in Q-switched mode. Optical pumps may be implemented in end-on as well as side-pumping configurations within a small mechanical housing (U.S. Pat. No. 8,576,885 issued on Nov. 5, 2013, to van Leeuwen et al., United States Patent Application Publication No. 20130255613 published on Oct. 3, 2013, by Hartke et al., United States Patent Application Publication No. 20140290613 published on Oct. 3, 2013, by Hartke et al., United States Patent Application Publication No. 20130199483 published on Aug. 8, 2013 by Herden et al., United States Patent Application Publication No. 20090133655 published on May 28, 2009, by Inohara et al., U.S. Pat. No. 8,322,320 issued on Dec. 4, 2012, to Oledzki, U.S. Pat. No. 6,382,957 issued on May 7, 2002, to Early et al.).
Most of the prior art laser ignition device use either a remote (using a fiber) or an integrated optical pump assembled in a cylindrical housing. In one prior art design the optical ignition is equipped with a spring loaded housing for a vibration free operation (U.S. Pat. No. 8,312,854 issued on Nov. 20, 2012 to Weinrotter et al.). Other design aspects include components for fixed or variable focusing optics such as lens or microlens array, beam guiding devices, window including sapphire window, coupling devices, polarization selective output option, apparatus to generate a plurality of individually addressable beams, an antechamber construction to pre-ignite the combustion fuel and introduce the hot plasma at many different locations of the internal combustion chamber, and provision for air or liquid cooled apparatus with heat exchanger for cooling the optical pump as well as the solid state laser material.
Despite all the progress made in innovative designs for laser ignition devices or optical spark plugs, there is still a need for an optical spark plug that would easily adapt to, and conform to the widely used electric spark plug with added advantage of ultra-compact small form factor, higher combustion efficiency, longer life cycle and lower environmental cost. In this invention a compact laser ignition device is provided. The apparatus as designed includes a solid state ignition laser, an optical pump source, beam shaping and steering optics in a robust and compact housing that may even be smaller than 3.0″ in length and about 0.75″ in diameter. The device as claimed functions for combustion of fluids or fluid mixtures including gasoline, and natural gas.